Coercivity distributions in Nd-Fe-B sintered magnets produced by the grain boundary diffusion process

The grain boundary diffusion (GBD) process is an effective process for enhancing the coercivity of Nd-Fe-B sintered magnets. When the GBD process is applied to a thick magnet, the magnet exhibits a coercivity distribution. Partial coercivity at the centre is the same as that of the original magnet, but the partial coercivity increases on going towards the magnet surface. In this paper, the effects of conditions of the GBD process on the characteristics of coercivity distributions were investigated. Surface coercivities increase with increasing amount of coatings, while diffusion depths are almost the same. The diffusion treatment temperature varied from 800 to 900 degrees C, and does not affect the coercivity distributions. A long-time diffusion treatment is effective in enhancing the inner coercivities, but a very long treatment causes a significant reduction in surface coercivity.